Abstract

M dwarfs are the most numerous stars in the Galaxy. They are
characterized by strong magnetic activity. The ensuing high-energy
emission is crucial for the evolution of their planets and the eventual
presence of life on them. We systematically study the X-ray and
ultraviolet emission of a subsample of M dwarfs from a recent
proper-motion survey, selecting all M dwarfs within 10 pc to obtain a
nearly volume-limited sample (˜90 per cent completeness). Archival
ROSAT, XMM-Newton and GALEX data are combined with published
spectroscopic studies of Halpha emission and rotation to obtain a
broad picture of stellar activity on M dwarfs. We make use of synthetic
model spectra to determine the relative contributions of photospheric
and chromospheric emission to the ultraviolet flux. We also analyse the
same diagnostics for a comparison sample of young M dwarfs in the TW Hya
association (˜10 Myr). We find that generally the emission in the
GALEX bands is dominated by the chromosphere but the photospheric
component is not negligible in early-M field dwarfs. The surface fluxes
for the Halpha, near-ultraviolet, far-ultraviolet and X-ray emission
are connected via a power-law dependence. We present here for the first
time such flux-flux relations involving broad-band ultraviolet emission
for M dwarfs. Activity indices are defined as flux ratio between the
activity diagnostic and the bolometric flux of the star in analogy to
the Ca II R'HK index. For given spectral type, these indices
display a spread of 2-3 dex which is largest for M4 stars. Strikingly,
at mid-M spectral types, the spread of rotation rates is also at its
highest level. The mean activity index for fast rotators, likely
representing the saturation level, decreases from X-rays over the FUV to
the NUV band and Halpha, i.e. the fractional radiation output
increases with atmospheric height. The comparison to the ultraviolet and
X-ray properties of TW Hya members shows a drop of nearly three orders
of magnitude for the luminosity in these bands between ˜10 Myr and
few Gyr age. A few young field dwarfs (<1 Gyr) in the 10-pc sample
bridge the gap indicating that the drop in magnetic activity with age is
a continuous process. The slope of the age decay is steeper for the
X-ray than for the UV luminosity.